1. Field of the Invention
The present invention relates to a phosphorescent material, and more particularly to a photoluminescent phosphorescent material containing an alkaline earth aluminate and method for making the same.
2. Description of the Prior Art
Photoluminescent materials are well known in the art. They provide substance in material whereby items “glow in the dark” after having been exposed to light, either natural or artificial. These items range in use from watch faces, novelty items and the like to safety features such as lighted exit signs. An important feature of such materials is their decay time, or resident time that will remain luminescent and visible when the surroundings that the material is used in become dark.
In recent years, it has become important to use such materials in exit way lighting, such that an emergency exit way can remain lighted even after electrical power has been cut. With this type of material, the exit lighting absorbs light energy or radiation from the ambient lighting within a stairwell, for example, and can remain photoluminescent for long periods of time after the electricity has been cut. Very bright materials generally comprise phosphorescent materials such that the ambient light that they provide is very bright for an extended period of time, such as 12–24 hours.
Historically, there has been considerable commercial interest in the optical properties of rare earth activated alkaline earth (AE) aluminates due to their suitability in a variety of applications. Significant growth has taken place in markets such as opto-electronics, telecommunications and optically active commercial products including architectural lighting, building products and way-finding systems. With respect to the applications in the artificial lighting and illuminated display technology areas, AE aluminate materials have recently become important due to their greatly improved persistent photoluminescence properties relative to existing phosphors based on zinc sulfide (ZnS) and related phosphors. Among the most actively studied and commercially useful aluminates are those based on the system SrO—Al2O3, in which a number of stoichiometric oxide compounds are formed and remain stable at room temperature. Several of these oxide phases become optically active when doped with certain rare earth metals (REM's). The optical emission is attributable to the modifications of the electronic structure that arise relative to the non-doped host aluminate crystal.
Of particular importance in the phenomenon of persistent afterglow phosphorescence or photoluminescence are the strontium aluminates that are activated with small amounts of rare earth-containing oxides and compounds. Very early work indicated that persistent photoluminescence was obtained by adding 2–8 mole percent Eu2O3 to an equimolar mixture of strontium carbonate (SrCO3) and aluminum hydroxide Al(OH)3. Subsequent work near the composition SrAl2O4 extended the concept of Europium additions to include all other rare earth metals (REM) including Dy, La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Tm, Yb, and Lu.
It has become common practice to chemically identify optically active inorganic materials such as the AE aluminates that have been doped by indicating the stoichiometry of the oxide phase followed by the dopant chemical identity. Thus, the above-mentioned materials are denoted in the literature as SrAl2O4:Eu,Dy or SrAl2O4:Eu,Dy,Pr, etc. Where indicated, these additional dopants serve as co-activators and in certain specific formulations they have been found to enhance the photoluminescence behavior of the base Eu activated strontium aluminate or SrAl2O4:Eu and several other Sr aluminates.
The precise quantum mechanical mechanisms that govern this behavior in the AE aluminates have not been completely and unambiguously identified at this time. However, there is sufficient experimental and theoretical evidence to indicate that the process of electron/hole trapping of photostimulated carriers due to the presence of dopant species gives rise to the long decay time phosphorescence observed in these materials.
For example, the photoemission extinction time that marks the cessation of afterglow phosphorescence was found to increase by a factor of 10 to 15 for SrAl2O4 that is co-doped with about 1.5 mol % of Eu and Dy or Eu and one ore more of the rare earth elements mentioned above. The extinction time is commonly defined as the time required for the afterglow photoemission to diminish to 0.032 millicandela per square meter (mcd/m2). This value, though somewhat arbitrary, is approximately 100 times the commonly accepted limiting light intensity that can be detected by the human eye.